If disturbance noise is superposed on a resolver signal input from a resolver to an RID converter, the operation of the R/D converter is affected by the disturbance noise, and the digital angle output of the R/D converter have an angle error due to the disturbance noise.
Methods of removing the noise superposed on the resolver signal have been already proposed. For example, in the patent literature 1 (Japanese Patent Application Laid-Open No. 2003-153496), use of a common mode filter inserted in the input stage of an R/D converter is described.
However, the method described in the patent literature 1 assumes that the motor line for supplying the motor current to the motor and the resolver line for drawing the resolver signal from the resolver that detects the angle of rotation of the motor are disposed in parallel with each other. The magnetic field produced around the motor line by the motor current traverses the resolver line to cause induced noise simultaneously in both the first and the second signal as differential signals. To eliminate the induced noise, which is common mode noise, according to the method described in the patent literature 1, a common mode filter is disposed in the input stage of the R/D converter.
Typically, the resolver is disposed close to the motor. Accordingly, the resolver is strongly affected by the magnetic field produced by the motor, and noise due to the magnetic field produced by the motor is superposed on the resolver signal. When the motor is rotating, the magnetic field produced by the motor is in synchronization with the rotation of the motor. Therefore, the superposed noise due to the magnetic field produced by the motor is low-frequency noise. Furthermore, the noise is normal mode noise and therefore cannot be eliminated by the common mode filter described above.
A basic configuration of the RID converter is disclosed in the patent literature 2 (Japanese Patent No. 3442316, Japanese Patent Application Laid-Open No. 2000-353957). However, the patent literature 2 makes no reference to removal of noise.